Heart

THE HEART
We begin this unit by examining the structural features that enable the heart to perform so reliably. We will then consider the physiological mechanisms that regulate cardiac activity to meet changing circumstances.
Blood flows through a network of blood vessels that extend between the heart and peripheral tissues. Those blood vessels can be subdivided into a pulmonary circuit, which carries blood to and from the gas exchange surfaces of the lungs, and a systemic circuit, which transports blood to and from the rest of the body. Each circuit begins and ends at the heart, and blood travels through these circuits in sequence. For example, blood returning to the heart from the systemic circuit must complete the pulmonary .circuit before reentering the systemic circuit.
Arteries, or efferent vessels, carry blood away from the heart; veins, or afferent vessels, return blood to the heart. Capillaries are small, thin-walled vessels between the smallest arteries and veins. Capillaries are called exchange vessels, because their thin walls permit the exchange of nutrients, dissolved gases, and waste products between the blood and surrounding tissues.
Despite its impressive workload, the heart is a small organ, roughly the size of a clenched fist. The heart contains four muscular chambers, two associated with each circuit. The right atrium receives blood from the systemic circuit and passes it to the right ventricle (little belly). The right ventricle discharges blood into the pulmonary circuit. The left atrium collects blood from the pulmonary circuit and empties it into the left ventricle. Contraction of the left ventricle ejects blood into the systemic circuit. When the heart beats, the atria contract first, followed by the ventricles. The two ventricles contract at the same time and eject equal volumes of blood into the pulmonary and systemic circuits.

I ANATOMY OF THE HEART
The heart is located near the anterior chest wall, directly posterior to the...